The present invention relates to a method of capacitive measurement between an object and an electrode plane. It finds its application in particular in the general field of 2D capacitive touch surfaces and 3D capacitive detection used for human-machine interface commands.
Increasingly, devices used for communication and for work use a touch command interface such as a pad or a screen. There can be mentioned for example mobile phones, smartphones, electronic notebooks, PC, mice, touch screens, widescreens, etc.
A large number of these interfaces use capacitive technologies. The touch surface is equipped with conductive electrodes linked to electronic means making it possible to measure the value of the capacitances created between electrodes and the object to be detected in order to perform a command.
The current capacitive techniques most frequently use two layers of conductive electrodes in the form of rows and columns. The electronics measure the coupling capacitances existing between these rows and columns. When a finger is very close to the active surface, the coupling capacitances close to the finger are altered and the electronics can thus locate the 2D position (XY) in the plane of the active surface.
This technology makes it possible to detect the presence and the position of the finger through a dielectric. This technique has the advantage of obtaining a very high resolution on the location in the plane XY of the sensitive surface of one or more fingers. These techniques nevertheless have the drawback of only detecting a contact with the object or a detection in very close proximity but not exceeding a few mm. It is difficult to perform touch commands with thick gloves (ski gloves, motorcycle gloves, etc.), with long fingernails or with a stylus. The low sensitivity of the capacitive electrodes does not allow a command to be initiated through a thick dielectric.
There are also more recent techniques allowing measurement of the absolute capacitance created between the electrodes and the object to be detected. This technique is similar to the techniques known as self-capacitance. There can be mentioned for example the patent FR2756048: Floating capacitive measuring bridge, patent FR2893711: Device and Method of Capacitive Measurement by a Floating Bridge or patent FR2844349: Proximity Detector Comprising Capacitive Sensor. These techniques make it possible to obtain a very high resolution measurement of the inter electrode-object capacitance and to detect for example a finger at a distance of several cm or even at a distance of 10 cm. The spatial detection takes place in 3 dimensions XYZ but also by touch within the plane XY. This time it is possible to initiate a command with a glove or through any type of thick dielectric.
These recent techniques involve placing a uniform conductive plane behind the measuring electrodes, brought to the excitation potential of the electrodes facing the system ground, optionally optically transparent, which eliminates the parasitic capacitive couplings between the system ground and the electrodes.
The conventional capacitive measurement techniques use a carrier frequency, preferably having a sinusoidal form, and a synchronous demodulation system for measuring the capacitance. When the nature of the sensor is purely capacitive, the measured signal corresponding to the inter electrode-object capacitance is in phase with the carrier. In practice, the nature of the touch surface is slightly resistive due to the resistivity of the material used, for example ITO for the transparent touch surfaces. This resistive nature of the sensor creates a phase offset which is seen by the electronics as a resistive leakage (in quadrature with the carrier) and which can create a significant error in the measurement. At the output of the system of measurement by synchronous demodulation, a quadrature error signal occurs that is proportional to the resistive leakage of the sensor. This signal results in degradation of the measurement of the inter electrode-object capacitance.
The aim of the present invention is to overcome the drawbacks of the prior art by proposing a novel method of avoiding this quadrature error.